1. Field of the Invention
This invention relates to automated apparatus for the calibration (standardization) of automated analytical equipment, wherein the analytical results are linearized with respect to the particular constituent being analyzed, the analytical measure of the constituent being nonlinear.
2. Description of the Prior Art
The prior art is replete with automated systems for the automated analysis of materials. Generally, these materials are in the liquid form, and are introduced into the apparatus as a continuous stream of successive samples. For example, one such apparatus is described in U.S. Pat. No. 3,241,432, issued to L. T. Skeggs and assigned to a common assignee. Such apparatus is operative to react each of the successive samples, in turn, and present such samples, in turn, to an analytical portion. Often the analysis is effected colorimetrically, the reaction of the sample producing a color product having an optical density which varies as a non-linear function of the concentration of the particular constituent under analysis. In colorimetric analysis, the output signal varies exponentially (antilogarithmically) with the concentration of the constituent, according to Beer's Law, as hereinafter discussed. Accordingly, to obtain meaningful analysis, the non-linear output signal of a colorimeter must be linearized by apparatus having a logarithmic input-ouput characteristic such that the reported analytical results vary linearly with the concentration of the constituent.
Also, analysis can be effected by the use of ion-selective electrodes, wherein the output signal varies logarithmically with the concentration of the constituent, according to the Nernst equation, as hereinafter discussed. In this case, the non-linear (electrode) output signal must be linearized by apparatus having an antilogarithmic input-output characteristic to effect linearization.
Besides linearization, it is essential that the analytical system be calibrated, according to the particular analysis to be effected, by proper relationship to a standard, i.e., a test sample of known concentration. Such calibration in prior art analytical systems required the manual adjustment of the system by the operator, which is not only time-consuming but introduces a source of human error. The more advanced analytical systems have sought to exclude or, at least, to minimize the intercession of the human operator within the "control loop" of the system. As can be appreciated, improper calibration of the system would produce inaccurate results which, for example, in the field of public health, could be disastrous. Elimination of the human operator, therefore, would reduce the possibility of error in the analytical results and, also, increase over-all efficiency of the system.
Techniques have been proposed for the automatic calibration and standardization of analytical systems, which have succeeded in substantially eliminating the human operator from the control loop. Also, such techniques allow for a continuing or frequent calibration of the apparatus, to provide accurate operation in the presence of drift. One such automated system has been disclosed in U.S. Pat. No. 3,681,577 to D. V. Gasiunas and assigned to a common assignee. This prior art system, however, is particularly operative to calibrate and standardize automatically those analytical systems having a response which varies linearly with the concentration of the constituent, for example, as is the case in spectroscopic analysis. Such system, however, is not particularly suitable in non-linear systems. Structurally, such system comprises an analog-digital processing system (including an A/D converter) wherein background-noise is translated to an equivalent DC signal and subtracted automatically from each of the analytical signals developed. Additionally, a variable gain is introduced into the processing system, the particular gain corresponding to the concentration range of signals under analysis, to amplify each analytical signal to an appropriate level for proper processing. The prior art signal processing system operates strictly on a linear basis. Fundamentally, such processing system is calibrated via a single standardization point (after subtraction of background) with respect to standards for each of the constituents and a digital scale factor is introduced to operate on each converted signal, so as to produce system results properly scaled in concentration units.